Our studies in the rabbit have shown that the young appendix is an important site of development of the cells destined to produce protective antibodies. The rabbit appendix has functions similar to those of the avian bursa of Fabricius. In the young rabbit, there is rapid growth and expansion of B lymphocytes in the appendix, and positive and negative selection events occur. We believe that the cells that survive selection in the appendix seed the peripheral lymphoid system and undergo self renewal to maintain the primary repertoire. The data on rabbit have led us to initiate an evaluation of the role of human appendix and GALT in development of the human immune repertoire and mucosal immunity. We started by asking at what age lymphoid development is most pronounced in the human appendix. Our conclusions are based on preliminary results of examinations 8-18 specimens by various stainings. Hematoxylin and eosin staining of paraffin sections of normal human appendix was first used to examine stages of germinal center (GC) development. At 4 days, only primary follicles were found. Lymphoid follicles appeared to develop rapidly within the first year of life and be maintained or gradually increased in size and number during childhood. Lymphoid development in appendix tissue from an 8 month old child resembled that from a 10 year old. In the samples from ages 4 months to 16 years, GC were evident with clearly defined light and dark zones. Lymphoid follicles had begun to atrophy in the sample from a 16 year old. No follicles were evident in one specimen from a 25-year-old. It appears that follicles begin to diminish in size and number during the teen years. The distribution of cells bearing common B and T cell markers at various ages was investigated by staining paraffin sections for IgM, IgD and IgA positive B cells and plasma cells and for CD4 and CD8 positive T cells. In a sample from a 4-day-old child, plasma cells (PC) were absent, anti-IgM stained most follicular cells whereas anti-IgD stained cells irregularly within the follicle and anti-IgA did not stain. Ig staining patterns were similar in the samples from 4 months to 16 years: GC were IgM+, mantle zones were IgD+, and GC light zones were weakly IgA+. CD4+ and CD8+ cells were found mainly in the T cell areas bordering the GC. Numbers of CD4+ and CD8+ cells varied greatly between GC from the same donor although T cells were found in the follicles from the earliest age examined. Do GC of the human appendix undergo the developmental changes seen in the rabbit appendix which are suggestive of transformation from a primary to secondary lymphoid organ? The distributions of B cells expressing IgM, IgD and IgA, and of T cells bearing CD4 appear similar throughout development as long as GC are present. It is uncertain whether this holds true for CD8+ cells. Double staining for possible double-positive cells is necessary and is in progress. Our finding of CD4 T cells in the germinal centers of developing human appendix suggests that it may function as a secondary lymphoid organ but does not exclude the possibility that it may also play a role as a primary lymphoid organ. The human appendix shares some characteristics of a "mammalian bursa equivalent"; the morphology of its follicles and the fact that the GC involute with age (Dasso et al., 1998 and ms in preparation).